1. Field of the Invention
The present invention relates to a printed circuit board and a manufacturing method of the printed circuit board, particularly, relates to a printed circuit board having a structure of transmission line, which is hardly affected by external noise, low in scattering of impedance and excellent in a transmission characteristic, and a manufacturing method of the printed circuit board.
2. Description of the Related Art
Recently, with entering into an informational age, equipment such as a communication terminal has been widely spread. Following this situation, equipment has been progressed in high frequency and a mobile terminal equipment of which radio frequency (RF) signal is in an order of GHz has been introduced in a market. Further, even in a memory subsystem of a personal computer, a higher speed processing has been advanced in a frequency of some hundreds MHz. In response to this trend, a printed circuit board being excellent in a transmission characteristic of which impedance is controlled in accordance with a higher frequency is demanded.
FIG. 5 is a cross sectional view of a printed circuit board according to the prior art.
FIGS. 6(a) through 6(d) are cross sectional views of a printed circuit board of the prior art for explaining a manufacturing process.
In FIG. 6(a), a copper foil formed on an insulative resin substrate 10 composed of glass epoxy, for example, is wet etched through a photolithography process and an inner layer circuit pattern 11 having a predetermined shape is obtained.
In FIG. 6(b), a surface treatment such as blacking process is applied on a surface of the inner layer circuit pattern 11. Insulative resin layer is formed on the insulative resin substrate 10 so as to cover the inner layer circuit pattern 11 by using a screen printing process or a curtain coating process, or by sticking a sheet material of insulative resin on the insulative resin substrate 10. Consequently, an insulative layer 12 is obtained. A thickness of the insulative layer 12 is 20 to 90 xcexcm. A blind hole, which reaches to the inner layer circuit pattern 11, or a through hole, which passes through the insulative resin substrate 10, is provided at a predetermined position on the insulative resin layer 12.
In FIG. 6(c), after roughening a surface of the insulative layer 12 with oxidizing agent, a copper layer is formed over the insulative layer 12 by the electroless plating method or electrolytic plating method, and then a conductive layer 13 is obtained. With respect to a degree of roughness on the surface of insulative resin layer 12, an average roughness Ra in the center line is approximately 0.1 to 20 xcexcm. Further, a thickness of the conductive layer 13 is from some xcexcm to some tens xcexcm.
In FIG. 6(d), an outer layer circuit pattern 14 having a predetermined shape is obtained by forming the conductive layer 13 through the wet etching process by the photolithography. Consequently, a printed circuit board having two layers of circuit patterns can be obtained. Further, in a case of multi-layering circuit patterns, it is accomplished by repeating the above-mentioned processes shown by FIGS. 6(b) through 6(d).
An impedance of transmission line in the printed circuit board mentioned above is mainly defined by a thickness and permittivity of insulative resin layer, a thickness and width of conductive layer of the transmission line and a configuration of grounding. Particularly, a strip line and an impedance of micro strip line are affected by a thickness of insulative resin layer and a line width of transmission line.
FIGS. 7(a) and 7(b) are ideal structures of transmission line and show a micro strip line and a strip line respectively.
In the micro strip line shown by FIG. 7(a), an impedance Zo1 of transmission line, which is composed of a first circuit pattern 1 for grounding, a first insulative resin layer 2 and a second circuit pattern 3 for signal formed over the insulative resin layer 2, can be given by a following formula (1).                     Zo1        =                              60                                                            ϵ                  ⁢                                      xe2x80x83                                    ⁢                  r                  ⁢                                      xe2x80x83                                    ⁢                  0.475                                +                0.67                                              ⁢          LN          ⁢                                                    5.98                ⁢                h1                                                              W                  ⁢                                      xe2x80x83                                    ⁢                  0.8                                +                t                                      ⁡                          [              Ω              ]                                                          (        1        )            
where xe2x80x9cxcex5rxe2x80x9d is a permittivity of the first insulative resin layer 2, xe2x80x9ch1xe2x80x9d is a thickness of the first insulative resin layer 2, xe2x80x9cwxe2x80x9d is a width of the second circuit pattern 3 for signal and xe2x80x9ctxe2x80x9d is a thickness of the second circuit pattern 3 respectively.
In the strip line shown by FIG. 7(b), an impedance Zo2 of transmission line, which is composed of a first circuit pattern 1 for grounding, a first insulative resin layer 2 and a second circuit pattern 3 for signal formed over the insulative resin layer 2, a second insulative resin layer 4 formed on the first insulative resin layer with covering over the second circuit pattern 3 and a third circuit pattern 5 for grounding, can be given by a following formula (2).                     Zo2        =                              60                                                            ϵ                  ⁢                                      xe2x80x83                                    ⁢                  r                  ⁢                                      xe2x80x83                                    ⁢                  0.475                                +                0.67                                              ⁢                      xe2x80x83                    ⁢          LN          ⁢                      xe2x80x83                    ⁢                                    5.98              ⁢                              xe2x80x83                            ⁢              h1                                                      W                ⁢                                  xe2x80x83                                ⁢                0.8                            +              t                                xc3x97                                    1                                                                    1                    +                    h1                                                                              h2ϵ                      ⁢                                              xe2x80x83                                            ⁢                                              r                        /                        ϵ                                            ⁢                                              xe2x80x83                                            ⁢                      r2                                        +                    h1                                                                        ⁢                          xe2x80x83                        [            Ω            ]                                              (        2        )            
where xe2x80x9cxcex5r2xe2x80x9d is a permittivity of the second insulative resin layer 4 and xe2x80x9ch2xe2x80x9d is a thickness of the second insulative resin layer 4 respectively. The other symbols are the same as those of the formula (1).
An RF of mobile communication terminal equipment such as a mobile telephone is mostly in a GHz band. An RF signal received by an antenna installed in the equipment flows through a circuit pattern for signal on a printed circuit board and is stepped down to some hundreds MHz in an intermediate frequency section. Finally the RF signal is stepped down to some tens MHz in a base-band. In order to obtain excellent receiving sensitivity of equipment, a transmission characteristic in a printed circuit board from an antenna to an intermediate frequency section is most important.
In order to prevent external noise deteriorating a transmission characteristic, a grounding line is provided in parallel to a circuit pattern for signal, which is a transmission line of printed circuit board. As shown in FIG. 5, a second circuit pattern 14a is formed above a first circuit pattern 11a for grounding being isolated by an insulative resin layer 12 provided between the first and second circuit patterns 11a and 14a. Circuit patterns 14b1 and 14b2 are provided along both sides of the second circuit pattern 14a in a predetermined distance.
On the contrary, the construction shown in FIG. 5 is insufficient in a shielding effect, so that equipment installing the printed circuit board is easily affected by an external noise and causes inferior receiving sensitivity.
Generally, an electromagnetic field in a high frequency can penetrate into a shallower depth from a surface of conductive material due to a skin effect. Consequently, a high frequency current flows through a surface layer of metal. A skin thickness xcex4 of surface layer, where a high frequency current flows through, is given by a following formula (3).                     δ        =                                            2                              μ                ⁢                                  xe2x80x83                                ⁢                σ                ⁢                                  xe2x80x83                                ⁢                ω                                              =                                    1                                                π                  ⁢                                      xe2x80x83                                    ⁢                  f                  ⁢                                      xe2x80x83                                    ⁢                  σ                  ⁢                                      xe2x80x83                                    ⁢                  μ                                                      ⁡                          [              m              ]                                                          (        3        )            
where xe2x80x9cxcexcxe2x80x9d is a permeability of conductive material, xe2x80x9c"sgr"xe2x80x9d is a conductivity of the conductive material and xe2x80x9cxcfx89xe2x80x9d is an angular frequency of an electromagnetic field respectively.
A circuit pattern for signal to be a transmission line is composed of a copper layer. If a signal frequency is 1 GHz, a skin depth is approximately 2 xcexcm. Consequently, it is apparent that a signal current just flows through a skin layer of circuit pattern.
FIGS. 8(a) and 8(b) are ideal drawings showing a current distribution of signal in a printed circuit board of the prior art. FIG. 8(a) shows a current distribution of signal in a lower frequency. FIG. 8(b) shows a current distribution of signal in a higher frequency. As shown in FIG. 8(a), a second circuit pattern 14a for signal is allocated on a first circuit pattern 11a for grounding being isolated by an insulative resin layer (not shown). A signal current in a lower frequency flows through the second circuit pattern 14a in a direction shown by an arrow xe2x80x9cisxe2x80x9d and a return current flows through the first circuit pattern 11a in a direction shown by an arrow xe2x80x9cirxe2x80x9d. Current lines indicated by a black dot xe2x80x9cxe2x97xafxe2x80x9d in the second circuit pattern 14a are uniformly distributed, and a current uniformly dispersed flows through the first circuit pattern 11a with utilizing the width of the first circuit pattern 11a maximally.
On the other hand, FIG. 8(b) shows a current distribution of a higher frequency signal of, for example, more than 10 MHz. A skin effect is remarkable and a current flow on an outer circumference area of the second circuit pattern 14a for a signal. In the first circuit pattern 11a for grounding, a current flows in a section under the circuit pattern 14a concentrically. In this case, the signal current tends to be concentrated at the section under the circuit pattern 14a and the return current tends to be concentrated at an upper section of the circuit pattern 11a. 
The skin effect is more remarkable if the frequency exceeds 1 GHz. In this case, a current is omnipresent sectionally if the second circuit pattern for signal is scattered and defective structurally. An impedance value being different from a predetermined value causes a large amount of scatter in impedance. Consequently, a high frequency transmission characteristic becomes unstable.
Accordingly, in consideration of the above-mentioned problems of the prior art, an object of the present invention is to provide a printed circuit board and a manufacturing method of the printed circuit board, which is hardly affected by an external noise and has a high frequency transmission line excellent in a transmission characteristic and low scattering in impedance.
In order to achieve the above object, the present invention provides, according to an aspect thereof, a printed circuit board comprising: an insulative substrate; a first conductive layer having a predetermined width and length being formed on the insulative substrate; a first insulative layer formed over the first conductive layer; a circuit pattern having a narrower width than that of the first conductive layer being provided in parallel with the longitudinal direction of the first conductive layer, wherein the circuit pattern is formed on the first insulative layer; and a second insulative layer formed over the circuit pattern, the printed circuit board further comprising: a plurality of grooves formed on both sides of the circuit pattern in the first and second insulative layers so as to expose the first conductive layer; and a second conductive layer formed on the second insulative layer continuously from an inner wall of one of the plurality of grooves to an inner wall of another groove out of the plurality of grooves so as to connect to the first conductive layer, wherein the second conductive layer connected to the first conductive layer through the first and second insulative layers forms a structure of surrounding the circuit pattern.
According to another aspect of the present invention, there provided a manufacturing method of a printed circuit board comprising steps of: forming a first conductive layer having a predetermined width and length, a first insulative layer, a circuit pattern having a narrower width than that of the first conductive layer provided in parallel with the longitudinal direction of the first conductive layer and a second conductive layer in order; forming a plurality of grooves on both sides of the circuit pattern in the first and second insulative layers so as to expose the first conductive layer; and forming a second conductive layer on the second insulative layer continuously from an inner wall of one of the plurality of grooves to an inner wall of another groove out of the plurality of grooves so as to connect to the first conductive layer, wherein the second conductive layer connected to the first conductive layer through the first and second insulative layers forms a structure of surrounding the circuit pattern.
Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings.